Conventionally, an electrophotographic system printer (image forming device) such as a copying machine and a printer includes a photosensitive drum (image holding body), a charging section, an exposing section, a developing device, a transcribing section, a fixing section, and the like.
In such a printer, a photosurface of the photosensitive drum is charged and exposed so as to form an electrostatic latent image, and the electrostatic latent image is developed by toner (developer) so as to form a toner image (visualized image). The toner image is transcribed to a sheet (recording medium: printing medium such as a plain paper, an OHP sheet, and the like), so as to be thermally fixed.
Further, it is known that the developing device of the printer includes a developing roller and a toner tank (toner holding tank) for storing toner. The developing device comes to the end of life when the toner runs out, and is replaced with a new one.
However, as to a former printer, maintenance of the developing device (replacement of the developing device) is performed by a specialized service person. Thus, it costs time and money to perform the maintenance, so that a user feels inconvenience.
Recently, a cartridge-type developing device (developing unit) that can be replaced (installed) by the user has been put into practical use, thereby facilitating the maintenance.
As described above, it is necessary to replace the developing device with a new one when the stored toner runs out. Thus, it is general that the developing device includes a toner remaining amount detection mechanism that detects an amount of remaining toner.
A technique relating to such a toner remaining amount detection mechanism is recited in Japanese Unexamined Patent Publication No. 84850/1999 (Tokukaihei 11-84850)(Publication date: Mar. 30, 1999) for example. FIG. 11 illustrates a toner container 200 recited therein.
As shown in FIG. 11, the toner container 200 includes a toner stirring member 201 for carrying toner stored therein to a developing roller. Further, (a) a rotating axis 202 for supporting the toner stirring member 201 and (b) a gear 203 that allows an external force to rotate the toner stirring member 201 are provided on one end side of the toner stirring member 201.
On the other end side of the toner stirring member 201, there is provided a slit plate 205 that passes through a photosensor 204.
The slit plate 205 is a disk having a plurality of slits. The photosensor 204 has a light emitting portion and a light detecting portion so that the slit plate 205 passes therebetween.
Therefore, the light detecting portion of the photosensor 204 is turned ON (receives light from the light emitting portion) when a slit of the slit plate 205 passes through the photosensor 204.
In the toner container 200, rotation of the toner stirring member 201 causes the toner stored therein to be stirred so that a part of the stirred toner is carried to the developing roller.
Further, when the toner stirring member 201 comes into the toner (toner pool) upon stirring the toner, the toner stirring member 201 receives resistance from the toner, so that a rotational speed thereof is reduced for a moment. Thus, a speed at which the slit plate 205 rotating with the toner stirring member 201 passes through the photosensor 204 is reduced, too. As a result, a cycle at which the photosensor 204 detects light (corresponding to a cycle at which the slit passes through the photosensor 204) becomes longer.
On the other hand, when the toner stirring member 201 is out of the toner, the toner stirring member 201 is suddenly released from the resistance of the toner, so that the rotational speed is increased for a moment. Thus, the speed at which the slit plate 205 passes through the photosensor 204 is increased. As a result, the cycle at which the photosensor 204 detects light becomes shorter.
Further, as more toner is stored (more toner remains), a period from (a) a time when the toner stirring member 201 comes into the toner to (b) a time when the toner stirring member 201 gets out of the toner becomes longer.
Thus, in the toner container 200, change of the cycle at which the photosensor 204 detects light is monitored, so that it is possible to estimate the amount of the remaining toner.
The foregoing toner remaining amount detection mechanism is required to rotate the toner stirring member 201 until the developing device including the toner container 200 comes to the end of life, that is, until just before the stored toner runs out. Thus, it is necessary that unwanted rotational resistance to the toner stirring member 201 (unwanted resistance: rotational resistance other than the resistance exerted by the toner stored in the toner tank) is kept at a low level for an extended period of time.
Incidentally, increase in the unwanted resistance to the toner stirring member 201 results from the toner which comes into a gap between the toner stirring member 201 and a roller bearing of the toner container 200.
Then, a ring member (not shown) is provided on the roller bearing of the toner container 200 shown in FIG. 11. The toner stirring member 201 is provided through the ring member, so that the gap is sealed, thereby preventing the toner from coming therein.
However, this configuration brings about such a problem that: when the ring member tightly holds the toner stirring member 201, initial unwanted resistance (resistance exerted by the ring member) to the toner stirring member 201 is increased.
Further, it may be effective that an inner surface (sliding surface) of the ring member is coated with resin which exerts little frictional force to the toner stirring member 201 so as to reduce the resistance exerted by the ring member. However, since the sliding surface is the inner surface of the ring, it is difficult to coat this appropriately, so that a manufacturing cost is increased.